The present invention relates to an automatic focus detection method, an automatic focus detection apparatus and an inspection apparatus for optically examining any micro-pattern defects and infinitesimal foreign matters that may occur on wafers and reticules as well as on masks and substrates during the process of fabricating flat panel displays such as TFT displays.
Conventionally, micro-patterns formed on wafers and the like are visually inspected by microscope optics having object images detected, magnified, projected and suitably processed. With microscope optics, the depth of focus (DOF) on the side of the object is determined by the wavelength .lambda. of illuminating light and by the numerical aperture (NA) of an objective lens in use. If a sample is positioned outside the depth of focus, a defocused image is detected. The depth of focus (DOF) is defined by the expression (1) below. EQU DOF=.lambda./(2NA2) (1)
One conventional technique for automatic focus detection whereby the surface of a sample is set to the focus of the objective lens is disclosed illustratively in Japanese Patent Laid-Open No. Hei 8-240765. One disadvantage of such a conventional technique is that a focus detection error is likely to occur due to changes in the reflectance of sample surfaces and in the density of patterns inspected.
Another disadvantage of the conventional focus detection method is that it tends to defocus in the presence of a stagger on the sample surface. That is, where the sample surface has a stagger that is included in the field, the detected focal point is given as a mean value between staggered surfaces. If the mean value exceeds a threshold depending on the stagger dimensions, the staggered surfaces of the sample are both outside the depth of focus DOF of the objective lens. The result is a defocused image. For example, the image of the object is focused correctly for depths of focus of up to about 0.3 .mu.m. Beyond that depth, the image is liable to be defocused.
Today's need to speed up inspection tends to increase the length of a linear sensor used for image detection. With the linear sensor prolonged, however, the field of the automatic focus detection system is still limited. This makes it difficult to detect a true focal plane.
Where samples with diverse spectral reflectance characteristics are subject to automatic focus detection, conventional techniques require setting up a plurality of light sources for generating spectra that are different from one another. The requirement tends to enlarge necessary equipment and pushes up costs.